Multiband filters for a plurality of mobile radio bands having different frequency ranges require either one wide or a plurality of narrower passband ranges in order that the different frequency ranges assigned to the bands can be permitted to pass.
The patent specifications U.S. Pat. No. 5,815,052, U.S. Pat. No. 6,043,725 and U.S. Pat. No. 6,115,592 disclose multiband filters which consist of a parallel circuit comprising two bandpass filters. U.S. Pat. No. 5,184,096 discloses a wide-band passband filter including a parallel circuit formed by three bandpass filters. Further multiband filters are obtained through specific design of the corresponding filter circuits.
The document EP 1 347 573 A1, also published as U.S. Pat. No. 6,927,648, discloses a multiplexer which includes high-, low- and bandpass filters and distributes input signals in four different frequency ranges correspondingly among four separate outputs.
One problem with known multiband filters is that a wide passband can be obtained only at sufficient distance from a stop band or, to put it another way, known multiband filters only have a transmission range with a flank that is flattened toward the stop band, with the result that these filters have an excessively large transition width between stop band and first passband.